An allergy to exercise might sound like a couch potato's dream, but for Kasia Beaver, a mom of four who lives in the UK, it's a nightmare.

Whenever her heart beats too fast and she sweats, it sparks a potentially fatal allergic reaction. Her eyes swell shut, she breaks out in hives and her throat closes up.

"It's terrifying, especially if I'm alone with the children," she said.

The 33-year-old is just one of a handful of people who suffer from a life-threatening allergic reaction to exercise known as exercise-induced angioedema – or EIA. The condition and a less severe form of exercise allergy called exercise-induced urticaria are so exceedingly rare, there are no estimates for the number of sufferers.

But Dr. Dennis Cardone, a sports medicine specialist at the NYU Langone Medical Center in New York, said exercise allergy is a real condition.

"Typically we see it in combination with another type of allergy, usually to food or hot weather," Cardone said.

As Cardone explained it, people with the condition may be able to eat peanuts or shellfish with no reaction and they may also be able to exercise with no problem. But if they eat peanuts or shellfish before they exercise, it triggers an attack.

EIA doesn't appear to have any association with exercise-induced asthma or any other type of breathing or exercise-related disorders. Cardone said the chemistry is the same as for most other allergies. The immune system mistakes a harmless substance for a dangerous intruder and responds by producing antibodies. These trigger special "mast cells" to produce chemical "histamines" that react with the rest of the body to produce the physical symptoms of an allergy.

An allergy to exercise generally isn't considered life-threatening. However, in Beaver's case it is. Doctors have told her that running for a bus or chasing after her children could kill her.

Oddly, Beaver didn't suffer her first attack until she was in her early 20s, just before she became pregnant with her first child, now 12. At first she thought it was an allergic reaction to a new eye shadow but after she stopped using it, her eyes remained swollen for three days.

One day she went to the gym and did her normal workout. Afterward her eyes felt tight. Her mother became so alarmed at her appearance, she immediately took her to the nearest emergency room. There doctors prescribed antihistamines that Kasia said helped, but the reactions continued and over time, became more severe.

"I was ice skating with my husband when I had a really bad attack. I had to use an epiPen to bring the swelling down," she recalled.

"It took me years to realize that exercise was the trigger."

Doctors were also baffled by Beaver's strange allergic reactions. When an eye specialist couldn't find anything wrong with her eyes, she was referred to a dermatologist. Finally after seeing one specialist after another she was diagnosed with EIA.

"It was a relief in a way because I could put a name to it. I wasn't going mad. I'd been tested for all sorts over the years. I thought it could be hay fever, a blocked tear duct or sweat gland," Beaver said.

Now, thanks to an antihistamine called ketotifen
, Beaver can walk to the park for the first time in 10 years without having an attack. She still can't exercise for any great length of time so she's joined a weight loss group in an effort to drop a few pounds.

"People don't believe me when I tell them I'm allergic to exercise. They think it's just an excuse to be lazy," she said.